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Article

Evaluation of Using an Octavius 4D Measuring System for Patient-Specific VMAT Quality Assurance

by
Yawo Atsu Constantino Fiagan
1,2,*,
Kodjo Joël Fabrice N‘Guessan
3,
Adama Diakité
3,4,
Komlanvi Victor Adjenou
3,4,
Thierry Gevaert
2,5 and
Dirk Verellen
1,6
1
Iridium Netwerk, Radiation Oncology, Oosterveldlaan 22, 2610 Antwerp, Belgium
2
Faculty of Medicine and Pharmacy, Vrije Universiteit Brussel, 1090 Brussels, Belgium
3
Department of Radiation Oncology, International Cancer Centre of Lomé, Lomé 6176, Togo
4
Faculty of Medicine and Pharmacy, University of Lomé, Lomé 6176, Togo
5
Department of Radiation Oncology, Universitair Ziekenhuis Brussel, 1090 Jette, Belgium
6
Faculty of Medicine and Health Sciences, Center for Oncological Research (CORE), Integrated Personalized and Precision Oncology Network (IPPON), Universiteit Antwerpen, 2610 Antwerp, Belgium
*
Author to whom correspondence should be addressed.
Radiation 2025, 5(1), 9; https://doi.org/10.3390/radiation5010009
Submission received: 18 December 2024 / Revised: 3 February 2025 / Accepted: 11 February 2025 / Published: 20 February 2025
(This article belongs to the Topic Innovative Radiation Therapies)
Browse Figures
Figure 1
<p>Linearity between the O4D detector system measured dose and Elekta infinity delivery monitor unit (MU).</p> "> Figure 2
<p>Results of output factor measured with the O4D detector array (in blue) and Semiflex detector (in red).</p> "> Figure 3
<p>%GP value of VMAT plans for simulated setup and delivery errors using 3%/3 mm and a 95% passing threshold. RS = right shift of couch, LS = left shift of couch, VS = vertical shift of couch, Rot = couch rotation angle, CR = collimator rotation, and MUv = monitor unit variation.</p> "> Figure 4
<p>%GP value of VMAT plans for simulated setup and delivery errors using 2%/2 mm and a 95% passing threshold.</p> ">
Versions Notes

Simple Summary
This study was to investigate the conditions in which pretreatment patient-specific quality assurance (PSQA) is performed, to evaluate the root cause of the implementation of more complex techniques, and to identify areas for potential improvement. In addition, the accuracy and sensitivity of the Octavius 4D system were investigated in detecting VMAT delivery and setup errors by measuring the variation in the percentage gamma passing rate (%GP) values before and after the simulated errors using only one VMAT plan for each treatment site. The results indicate that the Octavius 4D system is sensitive enough to detect delivery and setup errors with the restrictive global gamma criterion of 2%/2 mm for routine patient-specific pretreatment verification.
Abstract
Background: Quality assurance (QA) programs are designed to improve the quality and safety of radiation treatments, including patient-specific QA (PSQA). The objective of this study was to investigate the conditions in which pretreatment PSQA is performed, to evaluate the root cause of the implementation of more complex techniques, and to identify areas for potential improvement. Materials/Methods: The Octavius 4D (O4D) system accuracy was evaluated using an O4D homogeneous phantom for different field sizes. Tests of the system response to dose linearity, field sizes, and PDD differences were performed against calculated doses for a 6 MV photon beam. The pretreatment verification of 40 VMAT plans was performed using the PTW VeriSoft software (version 8.0.1) for local and global 3D gamma analysis. The reconstructed 3D dose was compared to the calculated dose using 2%/2 mm and 3%/3 mm, 20% of the low-dose threshold, and 95% of the gamma passing rate (%GP) tolerance level. The sensitivity of the O4D system in detecting VMAT delivery and setup errors has been investigated by measuring the variation in %GP values before and after the simulated errors. Results: The O4D system reported good agreement for linearity, field size, and PDD differences with TPS dose, being within ±2% tolerance. The output factors were consistent between the ionization chamber and the O4D detector down to a 4 × 4 cm2 field size with a maximum deviation less than 1%. The introduction of deliberate errors caused a decrease in %GP values. In most scenarios, the %GP value of the simulated errors was detected with 2%/2 mm. Conclusion: The results indicate that the O4D system is sensitive enough to detect delivery and setup errors with the restrictive global criterion of 2%/2 mm for routine pretreatment verification.

1. Introduction

Africa has a population of over 1.2 billion, comprising 54 countries, and cancer care services are limited in many of these countries [1]. There are 222 reported RT centers in 29 countries including Togo. The advanced external beam radiotherapy (RT) technique as volumetric modulated arc therapy (VMAT) enables optimal conformal dose distributions to the target volume (TV) while reducing the dose to organs at risk (OARs) [2,3]. The optimal dose distributions of VMAT are also attributed to photon beam modulation [4]. The VMAT technique can be achieved with one or more arcs by continuously varying the multi-leaf collimator’s (MLC) aperture shape, the fluence output rate, and the gantry rotation speed, which may yield a reduction in treatment time [5]. With the introduction of this complex technique, the inaccuracy, uncertainties, and errors in dose delivery have substantially increased. Moreover, geometrical and anatomical variations can also occur due to their complexity, and hence there are more possibilities for inaccuracies and uncertainties during the delivery of VMAT plans [6,7,8]. For these purposes, comprehensive quality assurance (QA) programs have been introduced including machine- and patient-specific QA (PSQA) to ensure the safety of patients, to prevent clinically relevant errors, and to improve accuracy. This PSQA can be performed prior to treatment (pretreatment verification) or during the treatment (in vivo dosimetry, IVD). Hence, the pretreatment PSQA for VMAT plans has become a current standard of practice [9,10,11,12]. Several studies have reported that different pretreatment PSQAs can detect different types of errors in VMAT delivery [13]. However, errors due to anatomical variations cannot be detected by mean of pretreatment PSQA. Thus, IVD is recommended as an additional method for PSQA [14,15,16]. For geometrical uncertainties, image-guided RT (IGRT) protocols have been introduced to mitigate these errors [17]. The most used clinical metric is the gamma index (γ) which treats dose distribution comparisons from a geometric perspective by evaluating the displacement between the measured and calculated dose distributions in terms of percentage dose difference (DD) and distance-to-agreement (DTA) [18,19,20]. Moreover, it combines the dose difference and distance difference to estimate a dimensionless metric for each point in the evaluated distribution [20]. Hence, the analysis results are reported as the percentage of points that achieved γ < 1 where the calculation passed both criteria, and γ >1 where the calculation did not meet both criteria [18]. Van Dyk et al. categorized the dose distribution comparisons into regions of high- and low-dose gradients, each with a different acceptance criterion [21]. In low-gradient regions, the doses are directly compared with an acceptance criterion relying on the difference between the measured and calculated dose which can be displayed through the visualization of their dose distribution, identifying the regions of disagreement. On the other hand, in high-dose-gradient regions, the concept of DTA is introduced to determine the acceptability of dose measurement because a small spatial error in either calculation or measurement results in a large dose difference and the concept of dose difference becomes irrelevant in these regions [18]. To address these challenges, low et al. have proposed a new approach that simultaneously incorporates both acceptance criteria, providing a numerical quality index referred to as the gamma value [18]. The latter can be defined as a measure of disagreement in the regions that fail and pass both criteria. Clinical protocols generally require a gamma pass rate (%GP) higher than 95% for analysis criteria with 3–5% DD and 3–4 mm DTA [20]. However, identifying a metric discriminating between passing and failing plans has been reported as a challenge. Several studies have compared various gamma index analysis and revealed that 3D gamma index metrics are more efficient because they provide a full volumetric assessment as an alternative to the single-plane 2D gamma index analysis [22,23,24,25].
The implementation decision of the PSQA system is based on studies of uncertainties associated with the level of training and expertise through the patient workflow including contouring, planning, and QA equipment. Thus, the PTW Octavius 4D (O4D) system and VeriSoft software version 8.0.1 (PTW, Freiburg, Germany) are used for this purpose due to a time-resolved dosimetric acquisition as the system rotates together with the gantry, allowing the reconstruction of a volumetric dose distribution for high-precision RT. The objective of this study was to investigate the conditions in which pretreatment PSQA is performed. Subsequently, we evaluated the root cause of the implementation of more complex techniques and identified areas for potential improvement.

2. Materials and Methods

2.1. Patient Selection and Ethics Statement

This study retrospectively analyzed forty patients who underwent RT treatment with the VMAT technique in our institute between January 2023 and June 2024. The study was conducted at the Lomé International Cancer Center (CICL) and approved by the ethics committee. Patient consent was waived for this PSQA. There were 10 VMAT plans each for patients with head and neck cancer (HNC; with prescribed dose of 70 Gy in 2 Gy for 35 fractions (fx)), uterine and cervical cancer (UCC, 70 Gy, 2.8 Gy/25 fx), prostate cancer (PC, 74 Gy, 2 Gy/37 fx), and breast cancer (BC, 50 Gy, 2 Gy/25 fx), the most diagnosed cancers in West Africa, particularly in Togo.

2.2. Treatment Workflow and Delivery Platform

All treatments were planned on the pinnacle treatment planning system (TPS, Philips Radiation Oncology Systems, Fitchburg, WI, USA, version 16.2.1) using the collapsed cone convolution (CCC) algorithm for dose calculation. All patients were treated with an Elekta infinity linear accelerator, equipped with Agility MLC, an electronic portal imaging device (EPID) system, IviewGT which is an MV imaging system, and an Elekta XVI® generally called a kV cone-beam computed tomography (kV-CBCT) imaging system. The dose was delivered using 6 MV photon beams with a VMAT technique. CT images were acquired with a 2.5 mm slice thickness, and TVs and OARs were automatically contoured using ART-Plan™ software (version 2.3.1, Therapanacea, Paris, France). The MOSAIQ record and verify system was used in transferring plan parameters to the linac control system. Online and offline IGRT protocols were applied to ensure a precise and reproducible patient setup. The daily online IGRT protocol included comparing orthogonal kV-kV images with digitally reconstructed radiographs generated from planning CT images for breast localizations and CBCT for others. Immediate corrective action prior to each treatment fraction was performed by automated adjustment of the treatment couch when the shift exceeded the action level of 3 mm. A couch override exceeding 2 cm cannot be unlocked automatically, so the RTT was obliged to inter to the bunker for manual adjustment. After the couch corrections, MV/kV or CBCT images was acquired for final verification. All plans were recalculated on the phantom with the same dose grid (3 mm).

2.3. Octavius 4D (O4D) VMAT Measurement System

The O4D measuring system is a combination of a phantom and detector 2D array 1500. The latter consists of 1405 cubic ionization chambers (IC) arranged in a plane-parallel configuration, forming a checkerboard matrix pattern on a 27 × 27 cm2 grid. Each individual ionization chamber covers a 4.4 × 4.4 mm2 area with a height of 3 mm, resulting in an active volume of 58 mm3 [26,27]. They are arranged in rows, and the center-to-center distance between the chambers in each row is 10 mm, and the distance between the rows is 5 mm. In addition to that, the center-to-center diagonal distance is 7.1 mm. This arrangement of the IC results in a spatial sampling frequency of 0.1 mm−1 in each row or column and 0.14 mm−1 in the diagonal direction. In this case, two measurements merged with a shift of 5 mm in the lateral or longitudinal direction, and the sampling frequency along each row and column can be doubled to 0.2 mm−1 [26]. The reference point specified by the manufacturer is located 7.5 mm below the surface of the detector array.
The detector array 1500 is accommodated inside the central cavity of the cylindrical phantom. This phantom is motorized and made of polystyrene (RW3), consisting of a water equivalent plastic with a density of 1.05 g/cm3, a diameter of 32 cm, and a length of 34.3 cm. It has a wireless inclinometer that transfers movement information to the phantom, and the latter acquires dosimetric data every 200 ms. The system allows for an insert for an ionization camber at the same position as the central chamber of the array. Data are processed by PTW VeriSoft software version 8.0.1 that allows dose evaluation with different metrics. The inclinometer is mounted on the gantry to ensure that the rotation unit always rotates along with the gantry, thus always keeping the 2D array perpendicular to the beam axis. The commissioning of O4D was carried with the same linac and limited to percentage depth dose (PDD) measurements and to the adjustment of phantom’s density in the TPS. PDD was measured for field sizes ranging from 4 × 4 cm2 to 26 × 26 cm2 measured at an 85 cm source-to-surface distance (SSD). The artificial O4D CT provided by the vendor was used for plan verification. To better model the phantom in the TPS, a CT scan of the O4D phantom was performed in the CT scanner and the obtained averaged Hounsfield unit (HU) was reported into the TPS. Furthermore, the exact distance from the couch was measured and reported in the artificial CT by the fusion of images [28]. The system was calibrated with a 10 × 10 cm2 field size with 259.3 MU corresponding to 2 Gy at central IC.
During plan generation, the center of the detector plane visible in the coronal CT slice was further marked by a reference point. From this point, a 2D region of interest (ROI) of a similar size as the detector area was extracted from the dose slice, resampled to the spatial resolution of the dose distribution measurement, and small spatial shifts caused by imperfect phantom setup were automatically corrected during gamma evaluation.

2.4. O4D Performance Tests

The O4D system dose linearity was tested by delivering 6 MV with a static field of 10 × 10 cm2 for the variety of MUs (2-600 MU) at gantry 0°. With VeriSoft software (version 8.0.1), doses were analyzed at the center of the 1500 detector array for each delivered MU and then normalized to the output for 100 MU. The Pearson correlation coefficient was evaluated for the linear correlation of doses vs. MU.
The field size dependence test involved 2 detectors, and dependence was evaluated by measuring the output factor (OF) for each field size ranging from 2 × 2 to 20 × 20 cm2 and normalizing with the 10 × 10 cm2 result. For each field size, 100 MU was delivered to the 1500 detector array and Semiflex ionization chamber (volume 0.125 cc, type 31010) in the same setup conditions. The output factors of both detectors were compared by plotting OFs vs. field sizes.
To compare dose variations between the O4D system and TPS, square field sizes ranging from 5 × 5 to 25 × 25 cm2 were planned with TPS using the O4D CT phantom. For each field size, the dose delivery was carried out with the gantry at 0° and 100 MU at the isocenter and different depths. The measured dose distributions were analyzed using VeriSoft software version 8.0.1, constituting the OF, dose profile, and PDD components for each field size. Then, these data were extracted from VeriSoft (version 8.0.1) by copying the displayed dose profiles at different depths of the detector array to text and importing into Microsoft Excel for analysis. Dose differences were then calculated relative to the center data point of the detector array. Dose distributions were analyzed with 2%/2 mm and 3%/3 mm acceptance criteria.

2.5. The Gamma Index and Error Detectability Analysis

The results of PSQA measurements were analyzed using the 3D gamma index with VeriSoft software (version 8.0.1) by comparing the reconstructed dose against the calculated dose from the TPS. This analysis was evaluated using 2%/2 mm and 3%/3 mm acceptance criteria. The center has adopted a global 3%/3 mm acceptance criterion as a clinical standard and a 20% low-dose threshold based on the results of the previous VMAT commissioning and on the type of dosimeter used. The mean and standard deviation (SD) of the gamma passing rate (%GP) values were calculated for each treatment site in all three planes (transversal, coronal, and sagittal) defined by the detector panel and evaluated with VeriSoft software version 8.0.1 for global and local absolute normalization.
To test the sensitivity of the O4D system, a single VMAT plan for each treatment site was used. Setup errors were deliberately introduced by shifting the phantom position away from the isocenter by 1, 5, and 10 mm in the left and right axial plane, by 5 mm in the vertical plane, and rotating the isocenter to 1–2° using the couch. The variation in MU and the collimator angle were also simulated by changing the rt-plan DICOM files throughout. The collimator was rotated from the isocenter by 1° and 5° and the output of the linac was varied using 2–3% MU. Gamma index analysis was evaluated with global 2%/2 mm and 3%/3 mm criteria by measuring the variation in the %GP value before and after the introduction of delivery and setup errors using the same MU for each VMAT plan, except in the case of MU variation error scenarios.

2.6. Statistical Analysis

A paired t-test was used to test for the significance of differences in mean %GP value between global and local GI analysis. A p-value < 0.05 was considered to be a statistically significant difference. The confidence limit (CL) was also evaluated based on mean and SD of %GP values for VMAT PSQA related to each treatment site using the following formula:
CL = (100 − %GPmean + 1.96 × σ% GPmean)
Therefore, the detection threshold (DT) corresponds to the difference between the expected value of 100%, and the CL was also evaluated as follows [29]:
DT = 100 − CL

3. Results

For the linearity between the dose measured with the O4D detector and the output of the Elekta infinity, three measurements were performed for each MU setting and their mean value was plotted against MU using the Pearson correlation approach. The R2 value was 0.999 with an SD of 0.3, showing that the dose response of the O4D detector array was linear as shown in Figure 1.
The output factors showed a similar trend between measurements using the O4D detector and PTW Semiflex detector. Differences are within 0.7% for 6 MV between field sizes from 4 × 4 to 20 × 20 cm2; for the small field size of 2 × 2 cm2, this difference is 1.3%. The results of the output factors for both detectors are represented in Figure 2.
For all static plans for field sizes from 5 × 5 to 20 × 20 cm2, the analysis results of %GP using 3%/3 mm and 2%/2 mm were greater than 98% and 93%, respectively, for global gamma index analysis compared to 97% and 85%, respectively, for local gamma index analysis. These results showed the decrease in %GP values for large static field sizes due to some mismatching in the penumbra region at the field edges. The comparison of agreement in the three different planes showed a better result on the transversal view for all static fields. The percentage dose difference (%ΔD) between the doses measured and calculated were within ±2% accuracy. All analysis results are reported in Table A1 (Appendix A).
The results of the PDD differences between the O4D system and TPS showed a maximum dose difference of 1.5% at a depth of 5 cm related to a field size of 10 × 10 cm2. The decrease in these PDD differences for each field size at different depths was also observed. The TPR20,10 was 0.63 at depths of 20 cm and 10 cm for a field size of 20 × 20 cm2, which was less than 1%. The results are reported in Table A2 (Appendix A).
The mean of %GP values and SD for 40 VMAT plans were evaluated. The gamma analysis evaluated with respect to the maximum point was significantly higher than that calculated point by point using 3%/3 mm for patients with PC (95.92% vs. 88.95%; p = 0.03), HNC (93.72% vs. 86.20%; p = 0.02), and UCC (93.46% vs. 84.05%, p = 0.01), BC (91.10% vs. 80.82%; p < 0.01), respectively. However, the acceptance criterion of 2%/2 mm was too tight, causing a significant decrease in the mean %GP value particularly for local gamma index evaluation. In cases of VMAT plan failure, medical physicists systematically reviewed the dose difference, distance-to-agreement, gamma index, isodose distribution, dose profile, and structure-specific dose distribution to determine whether the dose deviations are clinically relevant. Further analyses were performed to determine the root causes and reasons for these discrepancies in order to find an appropriate solution. For failed plans due to more complex modulation, replanning can be considered with less complex intensity patterns. The results evaluated on the three planes showed a better agreement on the transversal view for all VMAT plans, and the results are reported in Table 1.
The error detection threshold was evaluated only for global gamma index evaluation based on the mean %GP value, SD, and CL. The results showed that the DT for all treatment sites was higher than 85% for 3%/3 mm and 80% for 2%/2 mm. The results are reported in Table 2.
The simulated errors caused a decrease in %GP values in most scenarios. These errors were detected if the %GP of the simulated error was below DT or between the DT and %GP tolerance. However, the %GP value of the simulated errors above the passing tolerance was considered not detectable. The lateral shift of 1 and 5 mm, vertical shift of 5 mm, 1° and 2° couch rotation, 2° collimator rotation, and 2% MU variation were not detected in four cases using 3%/3 mm. By using the more restrictive criterion of 2%/2 mm, they can be detected except for the 1 mm shift and 1° couch rotation. All results are represented in Figure 3 and Figure 4, respectively.

4. Discussion

The accuracy and sensitivity of the O4D system were investigated for routine pretreatment verification at a new RT center in Togo. For this paper, two approaches were used to estimate the accuracy of the measured dose distribution in the homogeneous O4D phantom including the comparison of static field results and clinical VMAT plan results with the results of TPS calculation. Phantom measurements performed with the O4D system demonstrated very good linearity at the central axis for all of the delivered MUs (Figure 1). Moreover, the output factors were consistent between the ionization chamber and the O4D detector 1500 array down to a 4 × 4 cm2 field size with a maximum deviation of less than 1% which agrees well with the results of Stelljes et al. [30]. For all field sizes, the differences between the dose measured with the O4D system and the dose calculated with the TPS were within 2% accuracy. Thus, this ensures the correct implementation of the PDDs used for the reconstruction of 3D doses and accurate field size response of the O4D detector. It should be noted that the accuracy of the O4D system relies on the accuracy of the inclinometer to ensure that the detector array is always perpendicular to the incident beam.
An analysis of the results revealed that the mean %GP values for the global 3D gamma index analysis are relatively higher than the mean %GP values for local 3D GI analysis for all treatment sites. These results are consistent with the results reported by Urso et al. [28], who emphasized that the global normalization is calculated with respect to the value of the maximum dose and produced homogenous results with a higher %GP. Das et al. observed a significant correlation between the local and global gamma index and pointed out that it strongly depends on the dosimetric verification system and the resolution of detector used [31]. The results also indicated a dependence on the plane where the plan was evaluated and showed that the transversal view was linked to a better agreement if compared with the coronal and sagittal views. This behavior was already highlighted by Urso et al., who reported that the transversal view is most easily related to the treatment plan isodose on the transversal CT slice of the patient [28]. Our results coincide with a previous work of Esposito et al. [29], who evaluated dosimetric checks with O4D phantom measurements for 20 VMAT plans, including HNC and PC cases, and obtained a mean %GP (3%/3 mm) of 95.6% (±2.5%) with a 10% dose threshold.
The sensitivity of the O4D system in detecting deliberate errors from the dose delivery and patient setup was also investigated based on variations in %GP values and the detectability threshold in order to determine which criteria are most appropriate for routine pretreatment verification. The results indicated that the criterion 3%/3 mm with a threshold of 95% masked certain errors caused by deliberate MU errors of 2%, couch rotation angles of 1° and 2°, lateral (left and right) and vertical (up and down) shifts of 5 mm, and a collimator rotation of 2°. However, these errors were detectable using the more restrictive acceptance criterion of 2%/2 mm with a 95% threshold (Figure 2) except for a 1 mm couch shift and 1° couch rotation. For deliberate MU errors of 2%, these results were in close agreement with the results reported by Bresciani et al. [32], who emphasized in the clinical scenario that dose output variations in the VMAT plans of 1% or 2% could not be detected until a very strict gamma criterion of 1%/1 mm was applied. It was observed that the %GP value significantly decreased if 5% MU, 10 mm of left/right lateral shift errors, and 5° collimator rotation were simulated for the VMAT plan related to each treatment site. The O4D system was less sensitive for setup errors and delivery errors when the standard of 3%/3 mm is used. Moreover, setup errors can be corrected by using the IGRT protocol to mitigate the residual intra-fractionation displacements and rotations. Based on these results, the acceptance criterion 2%/2 mm could be relevant for routine pretreatment verification in our center.
However, the O4D system is limited to pretreatment verification and is not applicable to determine errors due to anatomical changes of the patient during the course of the treatment. Esposito et al. [29] and Mijnheer et al. [13] reported that the EPID-based IVD (EIVD) is the only tool able to verify the actual patient treatment, particularly with regard to patient anatomy and possible obstructions from positioning or immobilization devices. It can be used to assess and record the actually delivered patient dose over a series of treatment fractions without additional costs in measurement time and increase the synergy between RTT, medical physicists, and radiation oncologists. By performing EIVD, systematic morphological changes, due to tumor shrinkage, patient weight loss, and edema, could benefit from adaptive strategies [33,34]. Recently, Fiagan et al. demonstrated automated EIVD’s potential to identify changes in patient anatomy, patient setup, beam delivery, and imager position and to aid in triggering adaptive radiation therapy [34,35,36].
Our study has some limitations. Firstly, the dosimetric impact of the introduced errors was not quantified by using DVH data because the center did not have the license for this option. Secondly, the study is a single-institute study with a limited sample size, a multi-centered study involving multiple institutions in West Africa could be required to generalize the clinical applicability in PSQA for a high-precision RT.

5. Conclusions

The results indicate that the O4D system is sensitive enough to detect delivery and setup errors with a restrictive global gamma criterion of 2%/2 mm for routine patient-specific pretreatment verification. Moreover, this system should be used in combination with kV-CBCT to improve dosimetry accuracy and treatment reproducibility.

Author Contributions

Conceptualization, D.V., T.G. and Y.A.C.F.; methodology, Y.A.C.F. and K.J.F.N.; software, Y.A.C.F. and K.J.F.N., validation, D.V., T.G., A.D. and K.V.A.; formal analysis, Y.A.C.F.; investigation, Y.A.C.F. resources, A.D.; data curation, A.D. and K.V.A.; writing—original draft preparation, Y.A.C.F.; writing—review and editing Y.A.C.F.; visualization, D.V. and T.G. supervision, D.V. and T.G. project administration A.D. and K.V.A. funding acquisition, Y.A.C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

This study was waived because of a retrospective evaluation based on patient data, with approval to access the data provided by the Review Board of CICL (study number CICL01).

Informed Consent Statement

Patient consent was waived due to the retrospective study.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We sincerely thank the CICL for providing logistical support for this work. The authors would like to acknowledge the radiation oncologist, radiation therapists, and a medical physicist for their cooperation.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

RT (radiotherapy), VMAT (volumetric modulated arc therapy), OAR (organ at risk), QA (quality assurance), PSQA (patient-specific quality assurance), HNC (head and neck cancer), UCC (uterine and cervical cancer), PC (prostate cancer), BC (breast cancer), fx (fraction), O4D (Octavius 4D), TPS (treatment planning system), CCC (collapsed cone convolution), MLC (multi-leaf collimator), kV-CBCT (kilovoltage cone-beam computed tomography), MU (monitor unit), EPID (electronic portal imaging device), IVD (in vivo dosimetry), RTT (radiation therapy technologist), %GP (gamma passing rate), γ (gamma index), TV (target volume), DTA (distance-to-agreement), DD (dose difference), CTV (clinical target volume), PTV (planning target volume), MP (medical physicist), PDD (percent depth dose), SSD (source-to-surface distance), TPR (tissue phantom ratio), OF (output factor), IC (ionization chamber).

Appendix A

Table A1. Three-dimensional gamma analysis results for different static fields in three directions and percentage dose difference (%ΔD) at the isocenter.
Table A1. Three-dimensional gamma analysis results for different static fields in three directions and percentage dose difference (%ΔD) at the isocenter.
Field Size (cm × cm)Direction3D Global %GP3D Local %GP
2%/2 mm3%/3 mm2%/2 mm3%/3 mm%ΔD
5 × 5Transversal99.910099.71000.9
Sagittal99.810099.6100
Coronal99.810099.6100
10 × 10Transversal99.810098.899.90.3
Sagittal99.710097.699.8
Coronal99.710097.499.7
15 × 15Transversal97.899.995.898.6−0.6
Sagittal96.699.895.498.4
Coronal96.599.895.298.2
20 × 20Transversal95.298.785.797.8−0.8
Sagittal94.698.585.597.5
Coronal93.298.285.497.2
Table A2. Difference (%) in PDD at different depths and field sizes between the O4D system and TPS using a homogenous phantom.
Table A2. Difference (%) in PDD at different depths and field sizes between the O4D system and TPS using a homogenous phantom.
Field Size (cm × cm)PDD Difference (%) for Different Depths
5 cm10 cm15 cm20 cm
5 × 50.90.60.50.4
10 × 101.51.20.80.3
15 × 151.30.90.70.3
20 × 201.10.80.60.5
25 × 2510.90.70.6

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Figure 1. Linearity between the O4D detector system measured dose and Elekta infinity delivery monitor unit (MU).
Figure 1. Linearity between the O4D detector system measured dose and Elekta infinity delivery monitor unit (MU).
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Figure 2. Results of output factor measured with the O4D detector array (in blue) and Semiflex detector (in red).
Figure 2. Results of output factor measured with the O4D detector array (in blue) and Semiflex detector (in red).
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Figure 3. %GP value of VMAT plans for simulated setup and delivery errors using 3%/3 mm and a 95% passing threshold. RS = right shift of couch, LS = left shift of couch, VS = vertical shift of couch, Rot = couch rotation angle, CR = collimator rotation, and MUv = monitor unit variation.
Figure 3. %GP value of VMAT plans for simulated setup and delivery errors using 3%/3 mm and a 95% passing threshold. RS = right shift of couch, LS = left shift of couch, VS = vertical shift of couch, Rot = couch rotation angle, CR = collimator rotation, and MUv = monitor unit variation.
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Figure 4. %GP value of VMAT plans for simulated setup and delivery errors using 2%/2 mm and a 95% passing threshold.
Figure 4. %GP value of VMAT plans for simulated setup and delivery errors using 2%/2 mm and a 95% passing threshold.
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Table 1. Results of mean %GP and standard deviation using 3%/3 mm and 2%/2 mm acceptance criteria for different pathologies in our center.
Table 1. Results of mean %GP and standard deviation using 3%/3 mm and 2%/2 mm acceptance criteria for different pathologies in our center.
PathologyGamma IndexDirection2%/2 mm3%/3 mm
Mean ± SDMean ± SD
Prostate cancer (PC)3D globalTransversal92.87 ± 4.6596.43 ± 2.25
Sagittal91.79 ± 4.2695.79 ± 2.34
Coronal91.33 ± 4.1895.53 ± 2.23
3D localTransversal80.94 ± 5.1289.86 ± 4.89
Sagittal79.22 ± 5.4188.46 ± 4.67
Coronal79.13 ± 5.2088.52 ± 4.31
Head and neck cancer (HNC)3D globalTransversal91.45 ± 4.7994.85 ± 2.56
Sagittal90.23 ± 4.2193.25 ± 2.47
Coronal90.02 ± 4.1393.06 ± 2.43
3D localTransversal78.26 ± 5.8686.97 ± 4.95
Sagittal77.48 ± 5.5085.87 ± 4.81
Coronal77.03 ± 5.2185.76 ± 4.79
Uterine and cervical cancer (UCC)3D globalTransversal90.93 ± 4.7193.77 ± 3.75
Sagittal89.54 ± 4.2593.39 ± 3.36
Coronal88.74 ± 4.1793.23 ± 3.23
3D localTransversal75.94 ± 5.1284.86 ± 4.65
Sagittal74.22 ± 4.8483.76 ± 4.27
Coronal74.13 ± 4.2083.54 ± 4.21
Breast cancer (BC)3D globalTransversal89.94 ± 4.3293.65 ± 3.49
Sagittal88.76 ± 3.8692.59 ± 3.25
Coronal88.23 ± 4.2392.06 ± 3.22
3D localTransversal74.64 ± 5.7281.86 ± 4.76
Sagittal73.22 ± 5.4680.46 ± 4.37
Coronal73.13 ± 5.0980.14 ± 4.21
Table 2. Results of the confidence limits and threshold error detection for global analysis using 2%/2 mm and 3%/3 mm acceptance criteria.
Table 2. Results of the confidence limits and threshold error detection for global analysis using 2%/2 mm and 3%/3 mm acceptance criteria.
PathologyGAMean (%)SD (%)CL (%)DT (%)
PC2%/2 mm924.3616.5683.44
HNC90.574.3818.0181.99
UCC89.714.5519.2280.78
BC88.984.1419.1380.87
PC3%/3 mm95.922.278.5391.47
HNC93.722.4911.1588.85
UCC93.433.4513.3386.67
BC92.433.3214.0785.93
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MDPI and ACS Style

Fiagan, Y.A.C.; N‘Guessan, K.J.F.; Diakité, A.; Adjenou, K.V.; Gevaert, T.; Verellen, D. Evaluation of Using an Octavius 4D Measuring System for Patient-Specific VMAT Quality Assurance. Radiation 2025, 5, 9. https://doi.org/10.3390/radiation5010009

AMA Style

Fiagan YAC, N‘Guessan KJF, Diakité A, Adjenou KV, Gevaert T, Verellen D. Evaluation of Using an Octavius 4D Measuring System for Patient-Specific VMAT Quality Assurance. Radiation. 2025; 5(1):9. https://doi.org/10.3390/radiation5010009

Chicago/Turabian Style

Fiagan, Yawo Atsu Constantino, Kodjo Joël Fabrice N‘Guessan, Adama Diakité, Komlanvi Victor Adjenou, Thierry Gevaert, and Dirk Verellen. 2025. "Evaluation of Using an Octavius 4D Measuring System for Patient-Specific VMAT Quality Assurance" Radiation 5, no. 1: 9. https://doi.org/10.3390/radiation5010009

APA Style

Fiagan, Y. A. C., N‘Guessan, K. J. F., Diakité, A., Adjenou, K. V., Gevaert, T., & Verellen, D. (2025). Evaluation of Using an Octavius 4D Measuring System for Patient-Specific VMAT Quality Assurance. Radiation, 5(1), 9. https://doi.org/10.3390/radiation5010009

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